Gaston is an implementation of backward decision procedure for WS1S logic (weak second-order theory of k successors). The tool is using libmona a highly optimised deterministic finite tree automata library that supports semi-symbolic encoding using multi-terminal binary decision diagrams (MTBDDs) for storing transition table of the automaton. Procedure generates state space on-the-fly and prunes the states using the antichain-based approach. The tool works on the symbolical representation of the formula as Symbolic Automata and tries to prove on-the-fly that the initial states intersect the final states to (dis)prove validity.